Helicopters are fascinating flying machines. If you talk to someone with fixed wing experience about the effects of airspeed on a helicopter, they will look at you like you’re from a different planet. Fixed wing aircraft are for the most part inherently stable, while helicopters are inherently unstable. This means most answers to aerodynamics-related questions just create more questions. Most of these questions all come from the fact that airspeed does not affect all parts of the rotor disc in the same way, also called dissymmetry of lift. But what is it, and what are the causes and effects of dissymmetry of lift? That’s what we’ll be answering today!
We will split it up in the following sections:
What is Dissymmetry of Lift?
Unfortunately, unlike most fixed wing aircraft, a change in forward airspeed does not affect most of the aircraft equally. Helicopters have a giant rotating disc attached to them, which complicates things a little. To start out, imagine a counter-clockwise rotating disc in still air with a tip speed of 400 kts.
If we move forwards, the disc can be divided into the advancing side (blades move into the airflow across the disc) and the retreating side (blades move with the airflow across the disc):
Then, let’s say we build some forward airspeed to about 50 kts. This affects each side differently due to the blade travel direction. We add the 50 kts to the airspeed of the advancing side, and subtract it to the airspeed of the retreating side. That looks like this:
If we just left the disc like this, the advancing side would constantly generate more lift than the retreating side, and the entire helicopter would not stay level anymore, i.e dissymmetry of lift! Super annoying! Let’s talk about what we can do about this.
How do helicopters compensate for Dissymmetry of Lift?
We basically want the disc to correct for dissymmetry of lift by letting the blades flap (or sometimes slightly bend) up and down. Flapping is the vertical movement of a blade relative to the plane of rotation (the rotor disc).
The blade on the retreating side to flaps down, this increases the angle of attack:
Why does this happen? Well, a blade with less airspeed generates less lift, which results in the blade flapping down. When it flaps down it has more air hitting it from below, which increases the angle of attack, compensating for the reduction in airspeed.
The opposite is true for the advancing side. The blade flaps up due to an increase in airspeed, reducing the angle of attack, which compensates for the airspeed increase.
Do you remember the lift equation? We’re using TAS and Cl to equalise lift across the entire disc:
With these 2 variables compensating each other, we keep the disc level and ready to be used by pilot inputs without all these effects ruining our day. Tadaa, we have dealt with dissymmetry of lift! This technique is called flapping to equality. To summarise, it looks like this:
Notice that the point were the blade airspeed is the highest and lowest are the 9 and 3 o’clock positions. These are also the positions where the rate of flapping is the highest. In this case, the rate of flapping down is the highest at the 9 o’clock, increasing the angle of attack and therefore Cl the most at that point.
Now that we discussed where it comes from and what we can do about it, let’s have a deeper dive into the effects it causes on a flying helicopter. While flapping to equality is a nice way to solve dissymmetry of lift, it doesn’t solve all of our potential rotor disc problems..
What is Airflow Reversal?
Yea unfortunately we’re not done with all the aerodynamic shenanigans just yet. Airflow reversal, also called Reverse Flow, is another effect that needs dealing with that happens at higher airspeeds. Remember that we had to add the aircraft airspeed to the advancing side and subtract the aircraft airspeed form the retreating side?
Well, keep in mind that the highest blade airspeeds are present at the blade tips, while the blade roots are going much slower. This is because it covers a smaller distance in the same rotation as the blade tips. This means that when we subtract our aircraft airspeed from the root’s airspeed of the retreating side, we might end up with a negative airspeed value! What does this mean?
It means that the airflow is no longer going from leading edge to trailing edge, but instead goes the opposite way: from the trailing edge to the leading edge. At this point the blade isn’t doing much for us and just turned into a massive drag generator. There we have it: Airflow Reversal. The main thing we can do to limit airflow reversal is adhere to the maximim helicopter speed restriction: Vne.
Various aerodynamic tests have shown that some helicopters at Vne have roughly 40% of their retreating blade affected by airflow reversal. But it doesn’t stop here, this eventually causes Retreating Blade Stall as well, how? Let’s see!
What is Retreating Blade Stall?
So we have this retreating blade with a root area that doesn’t do much for us, reducing the total lift for this blade a lot. So what can be done to compensate for this?
Just like with dissymmetry of lift, a blade that generates less lift will naturally start flapping down more as it is no longer being pulled up by lift. The fact that we have this airflow reversal area present means the rate of flapping for this blade will increase even more.
This excessive flapping can result in blade stall. The rate of flapping will be higher at the tip as it has more travel than the root, so the initial stall tends to start at the blade tip rather than the root. Not only that, but airflow velocity impacts the stall angle as well. The graph below shows the relationship between TAS and the stall angle of attack. Basically, the higher the TAS, the easier it is for a blade to stall, which is why it tends to start at the blade tip.
So the retreating side has two potential hazards: airflow reversal and retreating blade stall. The risk for both of these increase as airspeed increases, and has a massive influence on the Vne of any helicopter. The density altitude, which we discussed here, and wave drag caused by reaching the speed of sound, have an influence as on Vne as well, which we discussed here.
The other risk factors besides aircraft TAS are excessive control movements, high G manoeuvres, and even flying in highly turbulent air. Retreating blade stall could and has caused accidents in the past, such as this HEMS BK117 B2 that lost 4000’ in altitude after a loss of control in the cruise phase.
So we’ve talked enough about all the problems, but what about the solutions? Let’s move on to:
How to recover from Retreating Blade Stall?
So what can we do about it? We first have to be able to recognise the symptoms: Rotor roughness, vibrations and random pitch and roll movements. This is more pronounced if we’re in a helicopter with a fully articulated rotor head compared to a rigid rotor head. This is because the freedom of movement of the blades could cause more excessive flapping motions. We do have rotor head and blade designs to limit the amount of flapping, such as Delta-3 Hinges and offset pitch horns, but that’s for another article.
Then to recover, our initial instinct might lean towards reducing airspeed right? The problem is that using aft cyclic when retreating blade stall is already present could worsen the problem. Any recovery should begin with reducing overall angle of attack across the blade: we have to lower the collective. As soon as the angle of attack has been reduced, it’s safe to reduce speed using aft cyclic.
Dissymmetry of lift has quite a few effects on helicopter and rotor disc design. It will always be one of the main downsides of having a rotor disc vs fixed wings. However, designs are still getting better and better every day, with higher Vne values across the board. Thank you for joining us today, and we’ll see you in the next article!